Been around with this for some time (please check closed issues):
The things is, that I want to inject my custom rendering in some pos (for example right before AA). Right now it means coping all parts over (https://github.com/armory3d/driver_celshade/blob/master/Sources/celshade/renderpath/RenderPathCreator.hx), adding and then keeping it manually up to date (with changes in Armory), what is highly unproductive, kind of dangerous.
If fragmenting and contextEvents isn’t the thing, maybe some other ideas will pop up, and can be discussed here. 
@lubos I really want to know your opinion on this. Do you don’t want the RP to be extendable ever? Or you have no time to think about it? Or you have some plan already? Im asking because I got no comments in this direction- just that my ideas will not work well. Like, Maybe brainstorming here has no point 
?